The present invention is concerned with a control valve, and in particular with a directly controlled proportional directional control valve for liquid or gaseous fluids.
Known directional control valves include a housing in which a valve bush is accommodated which guides a valve piston for axial displacement. The displacement of the valve piston is controlled by magnets so as to selectively link various flow connections between a pressure or inlet port, e.g. two working ports and a tank. Each working port is connected to the tank via a throttle to produce a dynamic pressure acting upon each end face of the valve piston in direction of displacement and thus supporting the force of the respectively actuated magnet.
The valve piston in such a directional control valve, is subjected to axial flow forces which oppose the force of the magnets, the magnitude of which is dependent on the rate of flow and the pressure drop at the valve.
The hydraulic output of such a directional control valve i.e. the maximal rate of flow at the respective pressure drop is limited by the maximal admissible actuation force of the magnets. Since these flow forces increase very rapidly with increasing rate of flow, they reach the magnitude of the magnetic force already at relatively small rates of flow so that the valve piston cannot be actuated any further thereby limiting the opening range of the valve.
It was proposed to provide a throttle between each of the working ports and the tank in order to generate a dynamic pressure which supports the force of the respectively actuated magnet. Such a dynamic pressure can, however, increase very rapidly so that the hydraulic force supporting the magnetic force exceeds the flow force and possibly an additional spring force. This, however, would urge the valve piston against the flow force and spring force against an end stop preventing any control or return of the valve piston into a neutral center position.